The present invention relates to a plastic shutter having at least two relatively thin face plates opposed to each other, a method for molding the plastic shutter, and a mold using the method. More particularly, the invention relates to a plastic shutter mounted on a cartridge case which accommodates a disc-like recording medium, specifically, a method for molding the plastic shutter and a mold for molding the plastic shutter.
With the advancement of plastic molding technology, plastic products have gradually superseded products formed of other materials.
A shutter of a disc cartridge is a typical example of a product which is now often made of plastic instead of another material (metal in this instance). Plastic shutters are disclosed in Published Unexamined Japanese Patent Application No. Sho. 60-231985, Published Unexamined Japanese Patent Applications Nos. Hei. 2-199685, 2-230580, 2-229019, 3-22276, 3-369077, and Published Unexamined Japanese Utility Model Applications Nos. Hei. 1-116620 and 3-81717. In such techniques, synthetic resin is molded to form the shutters for disc cartridges, which shutters were previously formed of thin metal plates, such as stainless steel plates. The manufacturing process (injection molding) used to produce resin shutters is simple compared with techniques used to produce metal shutters, and further is inexpensive to implement. That is, metal shutters require many manufacturing steps, such as bending and cutting steps, and require a high working accuracy.
A disc cartridge with which a plastic shutter is currently widely used is a 3.5-inch "microfloppy" disc cartridge constructed as shown in FIG. 11. In this microfloppy disc cartridge, designated by reference numeral 21, a magnetic disc 15 is rotatably disposed between rectangular upper and lower half shells 12 and 13 made of ABS resin, for example. The magnetic disc 15 is fixed to a disc center core 14. An opening 17 is formed in the central part of the lower half shell 13 of the cartridge 21. A rotary motor shaft engages the disc center core 14 through the opening 17.
A magnetic head insertion opening 18 is formed in both the upper and lower half shells. Through the opening 18, a magnetic head and a head pad move toward and away from the magnetic disc 15 for writing data signals into the magnetic disc 15 and for reading the same from the disc.
The microfloppy disc cartridge 21 further includes the above-mentioned shutter, designated by reference numeral 22. The shutter 22, which acts as an opening/closing member, is slidable to open and close the magnetic head insertion opening 18 for preventing dust from entering the microfloppy disc cartridge 21 through the opening 18 and attaching to the magnetic disc 15.
The shutter 22 is shaped like a U in longitudinal section. The shutter 22 is mounted so as to be slidable in the directions of opening and closing the magnetic head insertion opening 18 by a piece protruding toward the inner side of the shutter 22 and which is guided along a guide groove formed along the edge of the front end of the lower half shell 13. The shutter 22 slides with a slide area 11 including the circumferential edge of the magnetic head insertion opening 18 and reaching one side of the magnetic disc.
The structure of a mold for molding synthetic resin into such a shutter is constructed as shown in FIG. 12. As seen in this figure, slide cores 31 and 32 are disposed above and below (or on the right and left sides of) a plate-like member 30 located at the center. FIG. 12 is a longitudinal sectional view of the mold taken on a line D--D in FIG. 11.
An injection space 40 for shaping the shutter 22 is defined by the plate-like member 30, the slide cores 31 and 32, and the upper fixed mold 34. A portion of the shutter blank corresponding to the opening window 23 is formed by protruding portions of the slide cores 31 and 32.
Molten resin is injected into the injection space 40 through a gate 35 in the upper fixed mold 34. After the injected resin has properly hardened, the slide cores 31 and 32 are slid apart from the plate-like member 30 (in the directions of arrows C and D). Thereafter, the thus-molded shutter 22 is pushed outward by thrust pins provided in the plate-like member 30, separated from the plate-like member 30, and extracted from the mold.
The shutter 22 is thin, having a thickness of approximately 300 .mu.m to 400 .mu.m. The injection space 40 is thus an extremely small gap. Therefore, an extremely high injection pressure of 1000 kg/cm.sup.2 to 2000 kg/cm.sup.2 must be applied to the resin when it is injected into the injection space 40. The plate-like member 30 is a thin plate-like core having a thickness of approximately 2.3 mm to 3.0 mm. Therefore, if the plate-like member 30 receives a nonuniform resin injection pressure, it will be deflected.
The nonuniformity of the injection pressure can be due to the following reason (although it depends on the position of the gate 35 and the state of the flow of the injection resin). In the case of a mold having the gate 35 located in the upper part, a projection 44 as shown in FIG. 12, a projection 44 for forming a protruding piece 24 of the shutter 22 extends toward the plate-like member 30. Due to presence of the projection 44, the rate of flow (X) of the molten resin flowing through the channel having the projection 44 is smaller than the rate of flow (Y) of the molten resin flowing through the channel not having the projection 44. The flow rate difference causes the nonuniformity of the resin pressure.
Due to the nonuniform resin pressure, the shutter is convexly curved to the right side as shown in FIG. 12. When the deflection of the shutter is great, the plate-like member 30 deforms the gap of the injection space 40. Concerning the molding accuracy of the mold, if the gap difference of the injection space 40 is about 1 .mu.m, a pressure difference of about 1% will be induced. The plate-like member 30 is deflected to the length of about 50 .mu.m. The deflection causes a thickness difference between the right and left portions of the shutter in the injection space 40, and further makes nonuniform the thickness of each of the right and left portions of the shutter. To cope with this, the molding accuracy of the mold must be within .+-.0.1 .mu.m. It is difficult to attain this accuracy, and it is substantially impossible to maintain such accuracy during use of the mold.
Also with respect to the assembling accuracy of the mold, the dimensional differences caused within the injection space 40 result in a similar deformation of the mold. With the present state of molding technology, it is very difficult to reproduce the injection spaces 40 accurately by combining molds as described above at the accuracy of .+-.1 .mu.m or less.
When a thin member, such as the shutter 22, has a nonuniform thickness, the cooling velocity of the resin is also nonuniform, resulting in deflection of the shutter 22. The shutter, when deflected, can slide interruptively, and more adversely can damage the magnetic disc cartridge to an extent that the cartridge cannot be used. In the injection mode illustrated in FIG. 12, if the pressure difference of the injection pressure exceeds 3%, there is a danger that the plate-like member 30 will be destroyed.
For the above reasons, it is very important that the rein pressure of the resin injected into the injection space 40 of the mold be uniformly applied to both side walls of the plate-like member 30.
Presently, the following general methods of manufacturing the mold are available. In the first method, a mold is first manufactured and used to manufacture a shutter in a test run. The thickness of the side plates of the shutter molded by the mold is measured. Depending on the results of the measurement, the mold wall defining the injection space 40 is adjusted by cutting, for example. In another method, a mold is first manufactured as in the first method. The thickness of the side plates of the shutter molded by the mold is measured. Depending on the results of this measurement, a gas vent, provided for releasing gas within the injection space 40 when the resin is injected, is adjusted.
In these methods, after the mold is manufactured, the molding test is repeated for adjusting the dimensions of the mold or setting optimum values of the assembling positions. However, with these techniques it takes a long time before mass production of the shutters can begin, increasing the effective cost of the molds. Further, in the conventional methods, when, for example, the mold is disassembled for maintenance and assembled again, it is very difficult to set the assembly positions to the positions previously accurately set.